That volume, and not pressure, was the major factor was illustrated in an experiment in which the expansion of the lungs was limited by external binding and showing that despite high airway pressures, no injury ensued, compared with allowing a large volume to be delivered with unrestricted expansion, which resulted in injury.⁴¹ This has led to the use of low tidal volumes and higher breathing rates in order to provide “lung-protective” ventilation. Such a strategy has been applied in humans during surgery and appears to be associated with less inflammatory response⁴² and lower postoperative pulmonary complications.⁴³ In humans, the low tidal volume used is usually around 6 mL/kg. Using allometric scaling values, the normal resting tidal volume for the dog would be of the order of 8 to 10 mL/kg.⁴⁴ This would require respiratory rates of 25 to 70 breaths/min, with the higher rates being applied to smaller patients, to achieve normal minute ventilation.⁴⁵

With current practices of using very high inspired concentrations of oxygen, it is common to see atelectasis in dogs and cats after relatively short periods of anesthesia.^46,47 These areas of atelectasis create shear forces between the inflated and uninflated lung that can lead to tissue damage. This is exacerbated by the repeated cycling that is inherent to typical intermittent positive-pressure ventilation (IPPV). In conjunction with this, because the atelectatic areas are not expanding, the aerated parts of the lung will expand to a greater extent than normal, and this could lead to volutrauma in those regions. The management of this situation is to try to prevent the formation of atelectasis and to minimize ongoing airway collapse. The first approach is to apply positive end-expiratory pressure (PEEP) to the lung to prevent airway collapse.^47,48 It has been shown that even when high tidal volumes are used, the addition of PEEP decreases pulmonary injury.⁴¹ As PEEP is increased, it transmits pressure to the thoracic cavity, and this decreases venous return in a pressure-dependent manner.⁴⁹ If venous return is decreased, then cardiac output and oxygen delivery to the tissues will be decreased. At a PEEP of 5 cm H₂O cardiac output is reduced marginally, but at 20 cm H₂O, it is decreased by nearly 50% in dogs.⁴⁹ It is also thought that PEEP may have an effect on ventricular distensibility, further limiting cardiac output.⁵⁰ However, if the PEEP improves uptake of oxygen by decreasing atelectasis, it may improve oxygen delivery despite the reduced cardiac output. This has led to the concept of ideal PEEP whereby oxygen delivery is measured (cardiac output x oxygen content of blood) as PEEP is increased in a stepwise manner. The point at which oxygen delivery is maximized is the ideal PEEP under those particular conditions. Because it is not routine, in a clinical environment, to measure cardiac output, it would be difficult to make this titration. However, a correlation has been found for the difference between end-tidal (PE´CO₂) and arterial CO₂ (PaCO₂) tensions such that the point of lowest difference corresponds to “ideal” PEEP for that animal. This could be achieved by stepwise increases in PEEP and the measurement of serial blood gases at each step to compare the values for PaCO₂ with PE´CO₂.⁵¹ This approach may not work well when significant pulmonary pathology is present.

The second approach is to reduce the inspired oxygen concentration (F_IO₂) such that there is an insoluble gas, such as nitrogen, in the alveoli and lower airways that will not get absorbed and thus prevent absorption atelectasis. This is a really important factor because the combination of small tidal volumes and high F_IO₂s fosters absorption atelectasis. The addition of a nitrogen “splint” helps to reduce the likelihood of atelectasis. This has been demonstrated in both dogs and cats using an F_IO₂ of 0.4 and looking for areas of atelectasis with computed tomography (CT).^46,52 When doing procedures on animals when only one lung is being ventilated, there is invariably a decrease in the PaO₂.^53-55 If the lung that is being ventilated has some pathology, then this decrease will be greater, and it may be very difficult to decrease the F_IO₂ and maintain adequate oxygen delivery. Ideally, the animal should not be exposed to pure oxygen so that some nitrogen is maintained in the lung, but this is often unavoidable if hypoxemia is to be prevented during the manipulations required to isolate the lungs. If the animal has been exposed to pure oxygen, it may be necessary to apply a recruitment maneuver to minimize atelectasis. This term is very vaguely defined in the literature but essentially involves applying high pressures to the lung for a defined period of time. To sustain the opening of the airways, it is best if the recruitment maneuver is followed by the application of PEEP, a reduction in the F_IO₂, or both. In the past, many anesthesiologists have “sighed” patients by applying a 15– to 25–cm H₂O pressure for one breath every 5 to 10 minutes. This “recruitment” maneuver appears to be ineffective, and pressures of about 35 to 50 cm H₂O held for 20 to 60 seconds are needed to open the lung.^48,56 This may depend on the individual characteristic of the lung, but this author suggests starting with 40 cm H₂O held for 20 to 30 seconds.⁴⁸ This should be followed by a PEEP of at least 5 cm H₂O.

This is a noninvasive continuous monitor that estimates the saturation of arterial hemoglobin (SpO₂). It is an essential monitor for these cases because of the significant risk of desaturation associated with OLV. The reliability of these monitors is still not ideal in that they often give readings that are inaccurate, but in these cases, a low SpO₂ reading should not be disregarded with a supposition that it is just an erroneous reading.

The capnograph is also a noninvasive continuous monitor that provides essential information about the state of ventilation (PECO₂) and can also provide information about the circulation (e.g., sudden decrease in cardiac output) and the equipment being used (rebreathing indicated by a rise in inspired CO₂). In conjunction with the measurement of PaCO₂, it may also be used to optimize the ventilation strategy being used (see earlier).

Ideally, for these cases, this should be measured directly with the placement of an arterial catheter. This then provides a continuous measurement, allowing the anesthetist to see and respond to rapid changes in circulation.

With any intrathoracic procedure, there is a risk of manual compression or puncture of large vessels with a resulting sudden decrease in blood pressure. Noninvasive techniques (Doppler or oscillometric) may show such reduced pressures, but because the measurement is intermittent, they may not allow a rapid enough response to prevent serious consequences. A direct arterial line will also allow the measurement of systolic pressure variation (SPV) that is an indicator of hypovolemia. With the low tidal volume approach advocated earlier, the SPV has been shown to be predictive of fluid responsiveness.⁶¹ It is recognized that placing an arterial catheter is not feasible in all patients, so the most appropriate noninvasive technique should be used for these animals.

Electrocardiography is continuous and noninvasive and is the only monitor that allows the diagnosis of arrhythmias. Manual stimulation of the heart during surgery may precipitate an arrhythmia, and it will be beneficial to the patient if this can be accurately and immediately diagnosed and managed appropriately.

Although a pulse oximeter does provide a gross estimate of desaturation, if it occurs, it does not allow the accurate titration of F_IO₂ that is beneficial for these cases. Ideally, ABG analysis should be obtained from the catheter placed to measure blood pressure. This will provide the best information with regard to changes in oxygenation occurring with the onset of OLV. In dogs, it is well established that lingual venous samples can be used to approximate arterial values,⁶² if an arterial line cannot be placed. However, the relationship between lingual venous and arterial PaO₂ is not reliable on an individual sample, so this is not an ideal approach for these cases.

Several monitors now provide the capability of measuring flow and pressure of the gases during ventilation, allowing the machine to provide integrated outputs of pressure–volume and flow–volume loops and make calculations of compliance and resistance. These loops are helpful in looking at immediate changes in airway dynamics and in monitoring the expected changes in compliance associated with OLV. Such monitors usually have a function that allows a loop to be captured and remain visible on the screen. It is helpful to do this at each stage of the procedure so that changes over time can be compared with the starting point.

These cases often require imaging before the procedure, during which they may lose heat rapidly. Because hypothermia will decrease the HPV response, it is important to monitor body temperature throughout the procedure and provide external heat to maintain normothermia.

Ideally, the ventilator used for these cases should be volume limited and have a setting to allow PEEP to be added to the ventilation. Being able to use a volume rather than a pressure setting to control tidal volume allows the anesthetist to provide a protective ventilator strategy in order to limit the volutrauma to the lung. A pressure-limited ventilator can also be used in conjunction with spirometry to allow measurement and adjustment of the tidal volume. PEEP can be added in any circumstance by simply submersing the expired limb of the breathing circuit under water to the depth desired for the PEEP (e.g., submersing the tubing 10 cm under water will provide 10 cm H₂O of PEEP). However, it can be difficult to capture the gas from the fluid, so it is much easier if a PEEP valve can be attached to the expired side of the circuit. Such valves come in fixed increments (e.g., 2.5, 5 or 10 cm H₂O) and can either be spring-loaded or use a weighted ball. For the latter, the valve must be kept in the vertical position for the ball to seat properly in its mounting; the spring-loaded valves can be oriented in any direction. The valve must be connected in the direction specified by the manufacturer, or it may cause complete obstruction of the outlet, which could lead to death of the patient. Modern electronic and some older pneumatic ventilators may have a PEEP setting that allows the anesthetist to set an exact value in increments of 1 cm H₂O. This is obviously the most convenient approach because it can be adjusted very rapidly as determined by the clinical needs of the patient. If a recruitment maneuver is required, it usually needs to be done manually, so it is helpful if the ventilator can be switched in and out of the circuit rapidly.

Most machines provide the essentials of a variable flow of oxygen and a precision vaporizer to deliver the anesthetic. For these cases, it is also helpful to be able to vary the F_IO₂, so the addition of a compressed air source, an air flowmeter, and a method to measure the resulting concentration of oxygen in the mixture would be useful.